1. Field
The present disclosure generally relates to gyroscopes and also to MEMS structures and methods for fabricating a gyroscope with MEMS structures. More particularly, the present disclosure relates to a quartz-based disk resonator gyroscope.
2. Related Art
Mechanical gyroscopes are used to determine direction of a moving platform based upon the sensed inertial reaction of an internally moving proof mass. A typical electromechanical gyroscope comprises a suspended proof mass, gyroscope case, pickoffs, torquers and readout electronics. The inertial proof mass is internally suspended from the gyroscope case that is rigidly mounted to the platform and communicates the inertial motion of the platform while otherwise isolating the proof mass from external disturbances. The pickoffs to sense the internal motion of the proof mass, the torquers to maintain or adjust this motion and the readout electronics that must be in close proximity to the proof mass are internally mounted to the case which also provides the electrical feed-through connections to the platform electronics and power supply. The case also provides a standard mechanical interface to attach and align the gyroscope with the vehicle platform. In various forms gyroscopes are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
US Patent Publication 20070017287, “Disc Resonator Gyroscopes,” describes embodiments of a disk resonator gyroscope (DRG). In U.S. patent application Ser. No. 12/179,579, filed Jul. 24, 2008 and entitled “ALD METAL COATINGS FOR HIGH Q MEMS STRUCTURES” another embodiment of a DRG is described. In these disclosures the drive electrode and sense electrode are located internally to the resonator wafer. However, this location of the drive and sense electrodes requires that the resonator be coated with a conductive layer, which also requires that a thin conductive film be conformally coated within deep trenches of the resonator. A disadvantage of the conductive coating is that the Q of the resonator is degraded. Also this the process of conductive coating within the deep trenches introduces a chance of micromasking defects being created in the trenches.
Microelectromechanical systems (MEMS) technology relates to the small scale integration of mechanical structures with electronics. In general, MEMS structures are formed on a substrate using micromachining techniques in which layers are added to the substrate and in which material is selectively removed from the substrate and/or added. The electronics are formed using conventional integrated circuit (IC) techniques.
Resonators are fundamental to RF architectures and can be used as, for example, filters and oscillators, in MEMS devices. MEMS devices which consist of silicon-based resonators have been fabricated in an attempt to integrate nanoresonators or microresonators with other electronics. Nanoresonators and microresonators are resonators which have linear dimensions on the order of nanometers and micrometers, respectively. As used herein, ‘resonator’ refers to any resonator incorporated in, or coupled to, a MEMS device and includes both nanoresonators and microresonators. Silicon-based resonators have shown resonant frequencies as high as 600 MHz. However, a problem with silicon-based resonators is that they have high electrical impedances and low quality factors.
Quality factor (Q) is a measure of the frequency sensitivity of a resonate body and is related to how well a resonator traps oscillation energy. In other words, a resonator with a high Q exhibits low energy loss. Thus, the higher the value of Q, the better the resonator performance, and ultimately the better the performance of the overall MEMS device. A high Q factor is especially important for improving electrical functionalities in MEMS devices for RF applications.
One of the fundamental energy loss mechanism in a resonating solid is thermoelastic damping, which is caused by irreversible heat flows in the solid as it oscillates. The magnitude of energy loss depends on the thermal conductivity of the particular substrate material of which that solid is formed. Thus, for a given geometry and a fixed temperature, Q is strongly material dependent.
Because quartz can have lower intrinsic damping than other resonator materials, the use of quartz substrates in MEMS resonators is highly desirable. Quartz-based resonators are known to have a Q several orders of magnitude larger than silicon-based resonators with similar geometry.
For at least shear-mode piezoelectric resonators, thickness of the substrate determines resonant frequency. The thinner the substrate, the higher the resonant frequency. Techniques for thinning quartz substrates to obtain high resonate frequencies are known. However, another factor that influences how well oscillation energy is trapped is resonator surface smoothness. A rough or damaged surface increases a resonator's surface/volume ratio, which in turn increases dissipation of oscillation energy. Thus, when thinning a quartz substrate to obtain a high resonate frequency, it is desirable to maintain a smooth, undamaged surface to ensure a high Q.
U.S. patent application Ser. No. 10/426,931 for “A METHOD FOR FABRICATING A RESONATOR,” published as 2004/0211052 and now issued as U.S. Pat. No. 7,237,315, is co-owned with the present application and is incorporated herein in its entirety. The U.S. Pat. No. 7,237,315 patent is addressed to a method for fabricating a quartz nanoresonator that can be integrated on a substrate along with other electronics, disclosing methods for fabricating and integrating quartz-based resonators on a high speed substrate for integrated signal processing that utilize a combination of novel bonding and etching steps to form smooth, ultra-thin quartz-based resonators. The quartz resonators made in accordance with these methods can achieve a frequency in excess of 1 GHz.
U.S. patent application Ser. No. 11/881,461 for “AN INTEGRATED QUARTZ OSCILLATOR ON AN ACTIVE ELECTRONIC SUBSTRATE” is also co-owned with the current application and incorporated herein in its entirety. The Ser. No. 11/881,461 application discloses a method to attach a full wafer of quartz resonators to a substrate wafer using low temperature bonding, allowing a complete integration of a wafer of resonators to a wafer containing an array of oscillator circuits. As well, the Ser. No. 11/881,461 application also discloses integrating a quartz resonator with active electronics to form a low power, high performance (low vibration sensitivity, low temperature drift, and low phase noise) oscillator with reduced stray signals.
Quartz is an insulating material. Thus, to electrostatically drive or sense motion of a quartz-based resonator, the resonator's surface must be made conductive by forming an electrically continuous metal film thereon for electrodes. The thickness of the metal film, as well as the uniformity of that thickness, affects both resonate frequency and native Q of the resonator. The conductivity of the metal film affects RC time constants and impedances, which are critical for high performance in many MEMS devices. For example, electrically continuous thin metal coatings are desired for high frequency (>2 Ghz) oscillators and high performance quartz filters. The isolation and bandwidth of high performance quartz filters depend on making metal electrodes with ultra-thin films for optimal modal properties. For oscillators, a thick metal electrode reduces the frequency and can reduce the ultimate Q. Therefore, the best performing oscillators typically have very thin electrodes.
Deposition of metal coatings on electronic and MEMS structures has been restricted to the conventional thin film processes of sputtering, electron beam evaporation, or thermal evaporation. These processes are only slightly conformal (with sputtering being the best) on high-aspect-ratio structures, as these processes generally coat structures along a line-of-sight from the source, resulting in non-uniform thickness metal films which introduce stress gradients over temperature, degrading Q. Chemical Vapor Deposition (CVD) processes are more conformal than sputtering, electron beam evaporation, or thermal evaporation, but are not able to produce substantially pure metal films, desired for high conductivity.
Another problem with sputtering, electron beam evaporation, or thermal evaporation processes is the films generally form grain structures due to the kinetics of the grow process. Slippage between these grains during mechanical bending results in energy loss, degrading Q. Also, the grain structure limits the minimum thickness which can be applied in order to obtain electrical continuous films, also degrading Q as well as resonate frequency. The thickness of the metal film can not be controlled on the monolayer scale using these processes.
Atomic Layer Deposition (ALD) is a gas phase chemical process used to form extremely thin conformal coatings on a substrate. ALD is similar in chemistry to CVD, except that the ALD reaction breaks the CVD reaction into two sequential self-limited reactions between gas phase reactants (also known as precursors) and the growth surface.
Because ALD film growth is self-limited it is possible to control deposition on the atomic scale, as fine as approximately 0.1 angstroms per monolayer. ALD has unique advantages over other thin film deposition techniques, as ALD grown films are highly conformal, completely amorphous and chemically bonded to the substrate with essentially no grain structure. In addition, since the reactions only occur at the surface as opposed to in the gas phase as in CVD, no particulates are produced and the surface finish reproduces the original surface topology.
The use of ALD for ultra-thin film deposition of dielectrics has been used for several years in the IC industry, primarily in the fabrication of gate insulators and conformal passivation layers. However, successful demonstration of methods and chemistry to deposit pure metal films by Atomic Layer Deposition techniques was only first reported in 2005. (See Solid-State Lett. 8, C99 (2005). See also APL, Vol. 89, pp. 043111-1 to 043111-3 (2006).) In these ALD metal processes a metal precursor is introduced into a vacuum chamber. The metal precursor reacts with functional groups, usually oxygen, on the growth surface to deposit a monolayer of the desired metal. This precursor and oxidized hydrocarbon ligands from the precursors are then purged from the system. Next, a second precursor, such as oxygen, is introduced into the system. The second precursor combines with unreacted ligands on the surface and the metal to produce a clean oxide surface. Byproducts of water and carbon dioxide are produced, which are pumped away.
An all-quartz resonator gyroscope in accordance with the prior art is disclosed in U.S. patent application Ser. No. 12/179,579, filed Jul. 24, 2008 and entitled “ALD METAL COATINGS FOR HIGH Q MEMS STRUCTURES.” In that disclosure the drive electrode and sense electrode are located internally to the resonator wafer, as shown in FIG. 1A. This internal location of the drive and sense electrodes requires that the quartz disk resonator be coated with a conductive layer for the drive and sense electrodes. The conductive layer is a thin conductive film that is conformally coated within deep trenches in the system of interconnected rings on the disk resonator. However, a disadvantage of this prior art quartz resonator is that Q of the resonator is significantly degraded, when the conductive coatings are applied to all of the resonator portions.
What is needed is a quartz based disk resonator for a disk resonator gyroscope with improved Q. Since the bias drift of the gyro is inversely dependent on Q2, increasing the Q translates directly into less drift for the gyro. The embodiments of the present disclosure answer this and other needs.